Halo 3

The fights themselves are challenging, but the fact that you respawn at a Vita-Chamber everytime you die kinda takes away some of the challenge of the game overall, I think...
But you only have half your life and the feature is lost once you turn off the power, right? o.o At least, that's how it sounded in one of the gametrailers interviews. I got bored listening to the devs. XD

Yea, though. Does sound a bit less challenging than a checkpoint.